If you are talking about Visual FoxPro application (billing software) programming then you are the person who decides about the application behavior and you can program features which are not obvious in other commercial systems.

Visual FoxPro is so versatile product that it allows even such operations which could be considered as "illegal" under certain (accounting) circumstances...

So, everything depends on your needs and also on your skills. Some systems were even started as open source (http://projetosec.codeplex.com/) but I am not sure if it helps in this case.